Congress recently forbade President Obama from bringing Guantanamo inmates to the United States, or even preparing a place in the U.S. to which the terrorist prisoners might someday be transferred. The specific action Congress took was to renew earlier bans on the president spending any appropriated funds for those purposes.

Republicans and Democrats spoke in a strong and unified voice; 370 members of the House and 91 members of the Senate voted for the defense authorization bill that contained the Guantanamo provision. Other than a measure passed by unanimous consent, it's hard to find Congress more united.

Obama signed the bill into law, but at the same time released a signing statement making clear he might bring Guantanamo prisoners to the United States anyway -- no matter what Congress says.

"The restrictions contained in this bill concerning the detention facility at Guantanamo are ... unwarranted and counterproductive," Obama wrote. "As I have said repeatedly, the executive branch must have the flexibility, with regard to the detainees who remain at Guantanamo, to determine when and where to prosecute them, based on the facts and circumstances of each case and our national security interests, and when and where to transfer them consistent with our national security and our humane treatment policy."

Obama went on to claim that "under certain circumstances" the Guantanamo ban "would violate constitutional separation of powers principles." In those circumstances -- he didn't specify -- the president suggested he will use executive authority to move the inmates himself.

Republicans recalled 2008, when candidate Barack Obama promised he would not "use signing statements as a way of doing an end run around Congress." But hypocrisy aside, the substance of the conflict between Congress and the president could be the basis for a nasty and high-stakes fight between the branches of government.

"There is no ambiguity," a spokeswoman for House Speaker Paul Ryan said in response. "The president just signed a bipartisan bill into law that expressly prohibits him from transferring Gitmo detainees to the United States."

"This much is crystal clear," added Senate Majority Leader Mitch McConnell. "If the president wants to be able to import Guantanamo terrorists into Americans' backyards, he's going to have to persuade a majority in Congress to change the law."

Both Ryan and McConnell pointed to a recent House Judiciary Committee hearing in which Attorney General Loretta Lynch was asked about bringing Guantanamo prisoners to this country. "With respect to individuals being transferred to the United States, the law currently does not allow for that," Lynch testified.

It appears Congress -- not Republicans, not Democrats, but Congress -- is on solid legal ground. A former George W. Bush administration lawyer who asked not to be named argues that Bush, at the end of his presidency, "acknowledged that the Constitution directly provides Congress with power over the treatment and prosecution of detainees." Among those constitutional provisions are Congress' power to "make rules concerning captures on land and water" and to "define and punish ... offenses against the law of nations."

The Obama administration's position would be met with very strong, contrary arguments based on both of those constitutional provisions, and the Spending Clause (Congress' power of the purse), and the clause authorizing Congress to "make rules for the government and regulation of the land and naval forces," the former official said in an email exchange.

On the other side, in an op-ed earlier this month, former Obama White House counsel Gregory Craig pointed to the president's power as commander in chief and argued: "(Congress) can authorize detentions and military tribunals and broadly regulate the treatment of prisoners of war, but it cannot direct specific facilities in which specific detainees must be held and tried."

It's not clear whether that mostly unsupported assertion trumps 370 votes in the House and 91 in the Senate. (By the way, the latter vote would likely have been 97-3 had not several Republicans, including presidential candidates Marco Rubio, Ted Cruz, Rand Paul and Lindsey Graham, missed the vote.) As for Democrats, senators from Barbara Boxer, Tim Kaine and Barbara Mikulski on the left to Joe Manchin on the right joined Republicans in voting for the Guantanamo provision. That's a pretty broad coalition. (Just for the record, the three Democrats who voted against the measure were Bernie Sanders, Ron Wyden and Jeff Merkley.)

Barack Obama has exercised unilateral executive authority a number of times, but never in a case in which Congress was so clearly united against him. If the president decides to go it alone on the question of Guantanamo inmates, he might find that he has finally pushed things too far.